الفهرس 
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Abstract
Bacterial diseases affecting marine and freshwater fishes are numerous and they cause high economic losses in marine and freshwater culture sector in Egypt, therefore; this study was conducted for two main goals; Isolation and identification of the E. tarda isolates affecting cultured marine and freshwater fishes in Egypt as well as studying the relationship between heavy metal concentrations in water and pathogenicity test in both Oreochromis niloticus and sea bass with the incidence of these bacterial diseases .In our study, a total number of 100 cultured fishes (50 fish) O. niloticus and (50 fish) sea bass (D. labrax) . Fishes were collected showing clinical signs from brackish water fish farms at Wadi - Mariout districts in Alexandria governorates, Egypt during the summer period. The freshly dead fish specimens were subjected to full clinical, postmortem (PM) lesions and bacteriological examinations. In addition to, the different parameters of water quality namely; temperature, dissolved oxygen, unionized ammonia (NH3) nitrite (No2) and salinity (%) were measured during each sampling. The heavy metals in the water samples were also determined.
After culture on media and biochemical reactions, (14 isolates) of E. tarda from naturally examined O. niloticus and sea bass . Moreover, the results revealed that the external ulcer of sea bass was the most infected area by E. tarda (4 isolates), followed by liver (3 isolates), kidney (2 isolates) and the lowest infected organ is spleen (1isolates) . On the other hand, also the results revealed that the external ulcer of O. niloticus was the most infected area by E. tarda (2 isolates), followed by liver (1 isolates), kidney (1 isolates) and the E. tarda was not isolated from spleen.Results of the sensitivity of different E. tarda species isolated during this study : revealed the most isolates were susceptible to the majority of commonly used veterinary antibiotics against gram-negative bacteria including amikacin, amoxicillin, cefoxitin, ceftriaxone, ciprofloxacin, gentamicin, imipenem and streptomycin. The strains showed 100% resistance against colistin and trimethoprim / sulfamethoxazole. Intermediate resistance was noted against ampicillin and nalidixic acid. Most isolates displayed resistance against tetracycline, but three isolates presented intermediate resistance.* Histopathological results :In chronic of experimentally infected sea bass and O. niloticus were summarized as the following; Liver of O. niloticus experimentally injected with E. tarda showing degeneration of the pancreatic portion associated with leukocytic infiltration.
Liver of sea bass experimentally injected with E. tarda showing diffuse hepatic necrosis with perivascular and peripancreatic infiltration of macrophages laden bacteria.
Kidney of O. niloticus experimentally injected with E. tarda showing focal necrotic area associated with tubular necrosis and marked infiltration with mononuclear cells. Kidney of sea bass experimentally injected with E. tarda showing periglomerular infiltration with mononuclear cells mainly macrophages.